The third example concerns the eelgrass Zostera marina, which can reproduce both sexually and vegetatively. For species with two reproductive modes, molecular-genetic techniques are useful in assessing the relative importance of the two modes in determining population distributions and structure. Such information is relevant to many basic and applied questions in marine ecology because colonizers resulting from vegetative reproduction all have identical genetic composition, whereas sexual reproduction results in genetically diverse individuals. Genetic variation within and between populations of the same species may, for example, indicate the susceptibility of a given population to and its recovery from disturbance [ Grassle and Grassle, 1978].
Zostera marina is an important species both ecologically and economically because it provides critical habitat for a diversity of invertebrates and fish, it protects coastlines from erosion, it causes increased sedimentation leading to enhanced recycling of nutrients, and it improves water clarity. Because of the vigorous rhizomatic growth of this and other seagrasses, and their wide distributions and ecological success, vegetative reproduction has been expected to prevail. Thus, a high degree of genetic similarity within and even between not-too-distantly separated populations is expected.
However, using nuclear DNA fingerprinting to quantify genetic similarity of three geographically and morphologically distinct populations of Z. marina from central California, Alberte et al. [1994] showed just the opposite. Both within- and between-population genetic diversity was higher than expected for largely vegetatively reproducing individuals, even at scales as small as 5m. In addition, genetic diversity was lower for an intertidal population in a disturbed habitat than in a more pristine habitat only 30 km away, suggesting potential differences in the resiliency of these two populations.
This study indicates the value of molecular-genetic techniques for assessing ecological attributes of pivotal species within an ecosystem, and for making important management decisions. Zostera marina is one of several coastal macrophytes for which restoration projects---e.g., planting of seagrass meadows---are planned or in place, and whose success hinges somewhat on the supposition that the plants reproduce primarily vegetatively [ Zimmerman et al., 1991].